318 Tuning Tips - Help!

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Moparnut

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Hi guys.

I'll get this out before I go too far - I'm a panelbeater, not a mechanic, so please don't blind me with science. This is my setup -

Low compression 318,
Stock cam, dizzy (single points) and heads
New HT leads and plugs
Edelbrock 4BBL intake
Holley 600 (Vac Sec) manual choke
Headers with 2.5" dual exhaust - 1 muffler per pipe
904 Auto - standard

My problem - the damn thing is backfiring back through the carby. I have set the timing by ear to the point where it only just pre-ignites under real heavy load. A friend has adjusted the float levels in the Holley for me. The car is going pretty well now, but it has a real cough and backfire through the carb when I stand on its tail, before standing up and f**king off. Does anyone know what the cough and backfire means? :scratch:

Any help would be really appreciated.

Cheers!
 
Sounds like the timing is a bit high or the carb is too lean on jetting and needs to be jetted up some, make sure that you dont have a vacuum leak as this will cause this also.
 
You should really use a timing light...but I'd back the timing off a few degrees and test drive it. Have you messed with the acc pump squirter? The squirter cam? Might need a little fine tuning on the squirter and not the jets. I'd up the squirter a size or 2 then try it. Remember anything over .040 on the squirter size and you need the hollow screw. Also to set the PV you need to measure the manifold vacuum with the car idling in gear if it's and auto trans. Then subtract 1.5" from the reading on the gauge to get the PV you need. If it has back fired it's a good chance the PV is toast.
 
Try backing the timing down. Also, check manifold vacuum; if you have a rythmical dip you have a leaking intake valve. Use a timing light on the vac gauge to determine which cylinder, confirm with a compression test. Holley power valves don't like backfires so there is a good chance it is blown.
 
Awesome response, thanks guys. Now i'm gonna make myself look even dumber - what is a power valve, where is it and how do I replace it??

I'll get the timing light on it this weekend, but retarding the timing doesn't help any, I've tried all sorts of settings and it goes best where it is at the moment.

I don't have a vacuum leak that I know of, but I have a couple of small header leaks where they bolt up to the heads... could that be a problem?

Thanks again for the input! :)
 
The power valve is on the metering block behind the front bowl on the carb. It's right in the middle and it will take a 1" wrench to remove. It is a fuel enrichment valve. It opens at when the vaccume gets below a certain point hence the different valves.
 
Cool, thanks! We are taking it away on a car club road trip this weekend, so there is bound to be a bit of tinkering along the way. Hopefully by the end of the weekend i'll have enough knowledge to make some real progress with it. (That's if i can stay sober long enough to take anything on board! :evil2: ) Cheers!
 
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